In the future, when the number of users of cellular radio networks increases and rapid data transmission in these systems becomes increasingly common, an increase in the capacity of the system by improving the performance of the system becomes essentially important. One solution to this problem is the use of one or more adaptive antenna arrays instead of sector antennas. In an antenna array, single antenna elements are positioned typically close to each other, i.e. at about half a wavelength from each other. Typically, to facilitate the Fourier conversion, the number of antennas in such arrays is divisible by two and sufficiently large to achieve a desired coverage area. The basic principle of the method is to use narrow radiation beams that are directed towards the desired receiver as directly as possible. In the use of adaptive antenna arrays, the methods generally known can be divided into two main groups: directing radiation groups towards the receiver, or selecting the most suitable one of alternative beams. For the purpose of uplink transmission, a suitable beam is selected, or a beam is turned on the basis of the information received from the uplink. Reuse of frequencies can be made more efficient and the power of transmitters decreased, because interference caused to other users is reduced owing to the directivity of antenna beams.
The direction of antenna beams is typically implemented in a digital system by means of a digital beam formation matrix, for example a digital Butler matrix. A signal is divided in baseband parts into I and Q branches, and the signal of each antenna element is multiplied in a complex manner, i.e. phase and amplitude, by appropriate weighting coefficients, and after that, all output signals of the antenna elements are summed up. An adaptive antenna array comprises in this case not only antennas but also a signal processor, which automatically adapts antenna beams by means of a control algorithm by turning antenna beams in the direction of the most powerful signal measured.
A problem with generating antenna beams with a digital beam formation matrix of the prior art is that the phasing of antenna signals is performed as proportional relative to a reference antenna, in general the first antenna element in the array. Thus, the antenna elements in the array are phased relative to the reference antenna element but not relative to other antenna elements in the array. This leads to great power variations between the antenna elements in the array, which, in turn, leads to problems in the dimensioning of power amplifiers, for example in such a way that the power amplifier of one antenna element is much larger than the power amplifiers of the other antenna elements. Amplifiers that are powerful and as linear as possible are also expensive.
The directivity of beams can also be implemented analogically by generating orthogonal radiation beams by means of Butler matrices and fixed phasing circuits, in which beams the phase increases antenna by antenna. The method measures which beam receives the most signal energy, i.e. where the signal is most powerful, and this beam is selected for transmission. A problematic situation arises when the antenna beams are generated with a phase-shift network according to the prior art and the users of the radio network are spread unevenly over the areas of different antenna beams. The worst case possible is that all radio resource users are within the coverage area of the same beam, in which case in an antenna array with four antenna elements, quadruple power is required for one beam. Thus, the situation is the same as in a system with one antenna, so that array antenna gain is lost.